There are a variety of approaches to imaging in general. One approach is optical coherence tomography (OCT). OCT systems include Fourier domain OCT (FD-OCT) and time domain OCT (TD-OCT). FD-OCT generally includes swept source (SS) and spectral domain (SD), where SD systems generally use spectrometers rather than a swept source. TD systems generally rely on movement of a mirror or reference source over time to control imaging depth. In contrast, for FD-OCT, the imaging depth may be determined by Fourier transform relationships between the acquired spectrum, rather than by the range of a physically scanned mirror. Specifically, in FD-OCT, the number of samples of the spectrum may be used to control the imaging depth, with a greater number of samples of spectrum providing a deeper imaging capability.
In general, TD-OCT and FD-OCT are implementations of Low-Coherence Interferometry (LCI), a signal processing technique that relies on the mixing of two correlated broadband, or low coherence, optical signals that travel differential paths. Non-imaging implementations include Optical Low Coherence Reflectometry (OLCR), optical coherence-domain reflectometry (OCDR), and optical frequency domain reflectometry (OFDR).
Low coherence interferometry is a specific class of the more general concept of optical interferometry. There are many implementations of optical interferometers, including, for example, Michelson interferometers, Mach-Zehnder interferometers, and Fabry-Perot cavity interferometers. Michelson and Mach-Zehnder interferometers are commonly used for sensing, metrology, and imaging applications. Low coherence implementations rely on the concept of coherence gating. An interferometric signal whose modulation amplitude is proportional to the product of the two mixed signals is generated when the difference between optical path lengths is within the coherence length of the signal. The coherence length is inversely proportional to the source bandwidth.
Optical signals may be described by their state and degree of polarization. Typically, any optical signal may be decomposed into two orthogonal polarizations. The state of polarization (SOP) describes the relative amplitudes and phases of the two orthogonal components of polarization. The degree of polarization (DOP) describes the ratio of polarized light to the total irradiance. Unpolarized light is described by light that has energy distributed uniformly among the orthogonal polarization states, regardless of the basis set used to decompose the light field. Incandescent light tends to be highly unpolarized. In contrast, a laser tends to produce highly polarized light fields. A field may be described by the sum of its polarized component and its unpolarized component. A DOP of 1.0 describes a fully polarized field, and a DOP of zero describes an unpolarized field. Superluminescent light emitting diodes (SLED) used in OCT tend to have a DOP from about 50% to about 80%.
Light fields may be polarized by passing through a polarizer. Furthermore, light fields may be depolarized by a number of methods known to those having skill in the art.
Interferometric efficiency follows a cosine-squared law for the mixing of polarized signals. The signal strength of interfering polarized signals falls as the cosine-squared of the angle between the two polarizations. Orthogonal signals do not typically interfere. The reduction of interferometric efficiency caused by unmatched SOP can be referred to as polarization fading. Under certain conditions, unpolarized light interferes with a static reduction in polarization efficiency of about 50%.
Referring now to FIG. 1A, a conventional Michelson interferometer will be discussed. As illustrated therein, the interferometer includes an optical source 150, a beam splitter/combiner 101, first and second birefringent optical paths A 106 and B 107 and corresponding reflectors A 103 and B 104, and a detector 105. The optical source 150 may have an arbitrary coherence length and arbitrary DOP is incident on the beam splitter/combiner 101. A fraction of the signal travels the birefringent optical path A 106 towards the Reflector A 103. The remaining signal, ignoring some unavoidable losses, travels the second birefringent path B 107 to the second reflector B 104. The reflected signals from the Reflector A 103 and the Reflector B 104 reverse paths and recombine at the beam splitter/combiner 105, where the subsequent mixed signals interfere, and the interference signal is captured on the detector 105. In this configuration, no means for minimizing or controlling polarization fading is provided.
Referring now to FIG. 1B, a conventional Michelson interferometer using state-of-polarization (SOP) control to possibly reduce polarization fading will be discussed. As illustrated in FIG. 1B, the interferometer includes a polarized optical source 160, a beam splitter/combiner 101, first and second birefringent optical paths A 106 and B 107 and corresponding first and second reflectors A 103 and B 104, SOP control 102, and a detector 105. SOP control 102 may be used in one or both of the birefringent optical paths A 106 and path B 107. The use of SOP control 102 in this configuration may increase the likelihood that the polarization in one path may be aligned to the polarization in the other path, so that polarization fading may be reduced or possibly eliminated.
Referring now to FIG. 2A, a conventional Mach-Zehnder interferometer will be discussed. As illustrated in FIG. 2A, the interferometer includes an optical source 250, a beam splitter 208, first and second birefringent optical paths A 206 and 207, a beam combiner 209 and a detector 205. The optical source 250 may have arbitrary coherence length and arbitrary DOP is incident on the beam splitter 208. A fraction of the signal travels the birefringent path A 206 towards the beam combiner 209. The remaining signal, ignoring some unavoidable losses, travels the second birefringent path B 207 to the same beam combiner 209. The signals from path A 206 and path B 207 mix at the beam combiner 209 and the interference signal is captured on the detector 205. In this configuration, no means for minimizing or controlling polarization fading is provided.
Referring now to FIG. 2B, a conventional Mach-Zehnder interferometer using state-of-polarization (SOP) control to possibly reduce polarization fading will be discussed. As illustrated in FIG. 2B, the interferometer includes an polarized optical source 260, a beam splitter 208, first and second birefringent optical paths A 206 and 207, SOP control 202, a beam combiner 209 and a detector 205. SOP control 202 may be used in one or both of the path A 206 and path B 207. The use of SOP control 202 in this configuration may increase the likelihood that the polarization in one path may be aligned to the polarization in the other path, so that polarization fading may be reduced or possibly eliminated.
Referring now to FIG. 3, a conventional OCT system will be discussed. As illustrated in FIG. 3, the OCT includes a low-coherence or broadband source 300, polarization controllers 302, an isolator 305, a splitter/combiner 301 and a spectrometer. The low-coherence or broadband light source 300 is coupled to a splitter/coupler 301 by a source arm 308, a spectrometer 304 is coupled to the splitter/coupler 301 by a detector arm 303, a reference arm 306 extends from the splitter/coupler 301 to a reference, such as a mirror, and a sample arm 307 extends from the splitter/coupler 301 to a sample, schematically illustrated as a human eye in FIG. 3. In compensating for polarization effect in the OCT system of FIG. 3, it is known to install one or more polarization controllers 302 in the OCT system as illustrated in FIG. 3. These polarization controllers 302 may be used to increase the likelihood that the light returning from the reference arm 306 and the sample arm 307 are aligned relative to each other and potentially aligned with a dispersive element in the spectrometer 304.
The polarization controller 302 between the source 300 and an isolator 305 can be used to align the source polarization with the isolator 305, which may be polarization sensitive. Polarization insensitive isolators may also be used, in which case that polarization controller 302 between the source 300 and the isolator 305 may not be present. These polarization controllers 302 are typically “tweaked” or adjusted on an hourly or daily time scale in order to maintain optimal system performance. Such systems are typically sensitive to disturbances of any connections, but particularly those of reference/sample arms 306 and 307, the optical connections to the reference and the sample. System performance may also be sensitive to the performance of the broadband (Low-coherence) light source 300, the coupler 301 and the spectrometer 304.